After installing the on-line payment system (and making sure that it worked), we performed a routine upgrade of the Classified Ads feature. Unfortunately, that upgrade seemed to break the on-line payment system. So, we have disabled the Classified Ads feature and will re-install it in either late September or sooner–if we can resolve the conflict.
Sorry for the inconvenience or any frustration that it may have caused if you tried to pay on-line in early August.